[SCEV] rewriteLoopExitValues(): even if have hard uses, still rewrite if cheap (PR44668)

Summary:
Replacing uses of IV outside of the loop is likely generally useful,
but `rewriteLoopExitValues()` is cautious, and if it isn't told to always
perform the replacement, and there are hard uses of IV in loop,
it doesn't replace.

In [[ https://bugs.llvm.org/show_bug.cgi?id=44668 | PR44668 ]],
that prevents `-indvars` from replacing uses of induction variable
after the loop, which might be one of the optimization failures
preventing that code from being vectorized.

Instead, now that the cost model is fixed, i believe we should be
a little bit more optimistic, and also perform replacement
if we believe it is within our budget.

Fixes [[ https://bugs.llvm.org/show_bug.cgi?id=44668 | PR44668 ]].

Reviewers: reames, mkazantsev, asbirlea, fhahn, skatkov

Reviewed By: mkazantsev

Subscribers: nikic, hiraditya, zzheng, javed.absar, dmgreen, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73501

1 files changed, 7 insertions, 7 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopUtils.cpp b/llvm/lib/Transforms/Utils/LoopUtils.cpp
index 347289e..6902021 100644
--- a/llvm/lib/Transforms/Utils/LoopUtils.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUtils.cpp
@@ -1353,16 +1353,16 @@ int llvm::rewriteLoopExitValues(Loop *L, LoopInfo *LI, TargetLibraryInfo *TLI,
 
         // Computing the value outside of the loop brings no benefit if it is
         // definitely used inside the loop in a way which can not be optimized
-        // away. Avoid doing so unless we know we have a value which computes
-        // the ExitValue already. TODO: This should be merged into SCEV
-        // expander to leverage its knowledge of existing expressions.
-        if (ReplaceExitValue != AlwaysRepl &&
-            !isa<SCEVConstant>(ExitValue) && !isa<SCEVUnknown>(ExitValue) &&
+        // away. Avoid doing so unless either we know we have a value
+        // which computes the ExitValue already, or it is cheap to do so.
+        // TODO: This should be merged into SCEV expander to leverage
+        // its knowledge of existing expressions.
+        bool HighCost = Rewriter.isHighCostExpansion(
+            ExitValue, L, SCEVCheapExpansionBudget, TTI, Inst);
+        if (ReplaceExitValue != AlwaysRepl && HighCost &&
             hasHardUserWithinLoop(L, Inst))
           continue;
 
-        bool HighCost = Rewriter.isHighCostExpansion(
-            ExitValue, L, SCEVCheapExpansionBudget, TTI, Inst);
         Value *ExitVal = Rewriter.expandCodeFor(ExitValue, PN->getType(), Inst);
 
         LLVM_DEBUG(dbgs() << "rewriteLoopExitValues: AfterLoopVal = "